You're right about /10, but I think there is another morale-based reduction possible that is disabled only for units in high level forts or good defensive terrain (would have to check specific details).
Here are the full rules on it i think, along with the errata. I think there is another rule somewhere about isolated units being completely surrounded and being divided by 10 somehow if they fail a check.
(15.12.1) Isolated Units Combat Value Penalties
Isolated units suffer a supply related CV penalty that is equal to the percentage of needed supplies (or fuel for motorized units) times the percentage of needed ammo.
In addition, when calculating the modified CV ratio to determine whether a defender will be required to retreat, isolated defending units may have their CV divided by ten if they fail certain checks based on their morale, and the distance to the nearest supplied friendly units.
However, when defending units are in a hex with a defensive fortification modifier of five or greater (terrain plus fort level), then the above CV penalty does not apply. Instead, they undergo a check based on the defensive fortification modifier and their morale that may result in their CV being halved (CV halved if (random(25))/fort level
is greater than random (unit morale)).
(v1.03 Beta 3)
Isolated units will fire only ¼ as much as they would if they were not isolated in order to save ammunition.
1) It will be now possible for isolated units to attain airhead supply status, when enough supplies will be dropped to them, even without using standard airhead supply procedure via an air base.
2) CV of airhead supplied units or defending isolated units will not drop below 50% of original value due to resource shortages (was 33% and 25%).
3) In case of defending isolated or airhead supplied units, CV will drop only due to ammo shortages. Attacking isolated or airhead supplied units will still suffer from supply (non-motorized) or fuel (motorized) shortages.